feat: integrate DWN protocols with content and federation flows

- SCHEMA-03: content.add now writes DWN file-catalog/v1 message alongside
  the existing catalog entry. File metadata queryable via dwn.query-messages.
- SCHEMA-04: federation.join now writes DWN federation/v1 membership message.
  Federation relationships queryable via DWN protocol filter.

Both integrations are non-fatal on DWN errors (existing flows unaffected).

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

core/archipelago/src/api/rpc/content.rs

@@ -1,8 +1,11 @@
 use super::RpcHandler;
 use crate::content_server::{self, AccessControl, Availability, ContentItem};
+use crate::network::dwn_store::DwnStore;
 use anyhow::{Context, Result};
 use tracing::debug;
 
+const FILE_CATALOG_PROTOCOL: &str = "https://archipelago.dev/protocols/file-catalog/v1";
+
 impl RpcHandler {
     /// List content I'm sharing.
     pub(super) async fn handle_content_list_mine(
@@ -49,6 +52,36 @@ impl RpcHandler {
         }
 
         content_server::add_item(&self.config.data_dir, item.clone()).await?;
+
+        // Also store as DWN message for interoperable file catalog
+        if let Ok(store) = DwnStore::new(&self.config.data_dir).await {
+            let did = crate::identity::did_key_from_pubkey_hex(
+                &self.state_manager.get_snapshot().await.0.server_info.pubkey,
+            )
+            .unwrap_or_default();
+            let dwn_data = serde_json::json!({
+                "id": item.id,
+                "title": item.filename,
+                "description": item.description,
+                "content_type": item.mime_type,
+                "size_bytes": item.size_bytes,
+                "access": format!("{:?}", item.access).to_lowercase(),
+                "created_at": item.added_at,
+            });
+            if let Err(e) = store
+                .write_message(
+                    &did,
+                    Some(FILE_CATALOG_PROTOCOL),
+                    Some("https://archipelago.dev/schemas/file-entry/v1"),
+                    Some("application/json"),
+                    Some(dwn_data),
+                )
+                .await
+            {
+                debug!("DWN file catalog write (non-fatal): {}", e);
+            }
+        }
+
         Ok(serde_json::json!({ "item": item }))
     }